Thermionic device



Plaie Garrern Jan. 18, 1927.

Grid Voltage 1,614,462 H. M. DOWSETT THERMIONIC DEVICE Filed NOV. 12,1920 gmuawl'oz HARRY MELVILLE DOWSETT E751 his 3H awe/1 Patented Jam 18,1927.

UNITED STATES,

HARRY MELVILLE DOWSETT, OF COLCHESTER, CORPORATION OF AMERICA, OF NEWYORK, N.

ENGLAND, ASSIGNOR TO THE RADIO Y., A CORPORATION OF DELAWARE.

THERMIONIC DEVICE.

Application flled lwovember 12, 1920, Serial No. 423,711, and in GreatBritain November 22, 1919.

This invention relates to thermionic de vices such as those used inwireless telegraphy and telephony and commonly called valves. a

The cathode of such a valve is usually a good conductor and formed oftungsten, platinum or carbon.

According to this invention I construct a valve cathode of a refractorysubstance, particularly minerals such as carborundum, which are not goodconductors of electricity as compared with the usual materials used forlamp filaments.

I have found that a three electrode valve with a carborundum cathode hasa characteristic which differs markedly from that of valves havingcathodes formed of the usual conducting substances such as tungsten, orplatinum or these metals coated with the oxides of the alkaline earthmetals viz., cal cium, barium and strontium or the thoriated filaments;that is to say, as the grid voltage is raised from zero, the platecurrent rises very gradually until a certain critical voltage isreached, whereupon the plate current leaps up'to a very high value. Agrid voltage-plate current curve of these characteristics such asamplification or rectification is useful for many of the purposes forwhich valves are utilize While at the present time I prefer carborundumas the material of which I construct the cathode, I may use in placethereof any substance and particularly minerals having similarproperties such, for instance, as silicon or minerals such as zincite orbornite. The substance must be such that at the working temperature ofthe cathode it neither fuses nor disintegrates. In the claims mineralsand other substances within the scope of the invention will be referredto as refractory substances of low conductivity.

It is essential that when under working conditions, a current is flowingthrough the crystal, the junctions between the crystal and the twocurrent conducting wires should remain much cooler than the crystal. Thejunctions therefore require to be well made and to have as low anelectrical resistance as possible.

In the case of carborundum this result is obtained by welding on steeltips to the crystal to which the conducting wires themselves are welded,and in the case of other for a soft valve. In Fig.

crystals a welding metal is employed which has a melting point as nearas possible to that temperature at which the crystal changes either itsphysical or chemical nature.

For the same reason it is preferable that the crystal should be soshaped that it is thicker at the ends where the conducting connectionsare made than in the middle where it is required to reach the highesttemperature.

My invention is illustrated by the accompanying drawing in which Fig. 1illustrates a valve construction embodying the invention and Fig. 2is acurve showing the relation of the grid voltage to the plate current 1 Ais the crystal cathodeand B the welded-on metal tips by which thecrystal is connected to leads C.

Having now particularly described and ascertained the nature of my saidinvention and in what manner the same is to be per formed, I declarethat which I claim is 1. In a three element vacuum tube, means forincreasing the slope of the grid voltageplate current characteristicover that of tubes having cathodes of known materials and operatingunder similar conditions, said means comprising a cathode formed of amaterial having the electron emitting properties of any one of the groupof materials consisting of zincite, bornite, and carborundum.

2. In a three element vacuum tube in which the plate current is variedin accordance with variations in the grid voltage, means for rendering asubstantially larger change in plate current for a given change 7 ingrid voltage than is obtained in other vacuum tubes having cathodes ofknown ma terials and operating under similar conditions, said meanscomprising a cathode formed of a material having the electron emissionproperties of any one of the group of materials consisting of zincite,bornite and carborundum.

3. A thermionic valve having a cathode formed of a resistor elementcomprising a substance having the electron emission properties of anyone of the group of materials consisting of zincite, bornite, andcorborundum, said cathode having an intermediate portion of smallercross section than its end portions.

HARRY MELVILLE DOWSETT.

